Re: Strange sweep results

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Sundog wrote:
> got the expected Fres of around 833khz...But, ... ended up with
> another spike at 2.5mhz.

Something may be wrong here.

It's a little suspicious that this response is exactly 3 times the
normal Fres. Usually the 3/4 wave response is some 2.2 to 2.6
times the 1/4 wave Fres for an unloaded secondary. And the 5/4
wave is usually between 3.5 and 5.5 times the 1/4 wave.

Perhaps your coil just happens to have the right amount of
toploading to bring these two coil responses to a frequency ratio
of 3, which is possible (and may have unpleasant consequences for
CW operation).  Toploading reduces the frequencies of all the coil
resonances, but the lower frequency responses are pulled down more
than the higher frequency ones, so a ratio of 3 could be achieved
by chance.  It's just a little suspicious.

Is the 2.5Mhz reported from the sig gen frequency setting, or from
the observed period on the scope?  Whichever, can you check the
other?  Add a small amount of extra C to the coil top (eg a few
cm of wire or a door-knob) and re-measure.  If the 2.5Mhz is a 
genuine coil overtone, as opposed to a sig gen harmonic, the ratio
of the two frequencies should increase above 3.

> At harmonics I get a nice double-sine waveform, 
> with a big and small hump. 

Make sure you're using a sine wave drive.  The response will be
tricky to interpret if you use any other waveform.

>  this one is just as strong as the real Fres.

It is difficult to draw conclusions from the apparent difference
in the amplitude of the coil's responses.  So many factors are
involved:  Q factor of the coil, characteristic impedance
of the coil, probe/pickup response, sig gen output impedance;
All these have a direct effect on the observed amplitude, and all
vary quite a lot with frequency.

> Or should I try both and see which gives better performance?

The lower frequency is the one used for normal operation. The
higher overtones are very much less efficient at producing top
voltage because the effective capacitance of the secondary at
these higher frequencies is a great deal bigger - circa three times
larger for the 3/4 wave overtone than the normal 1/4 wave. Also,
when driven at the overtone frequencies, voltages equal to or
exceeding the topvolts will occur within the secondary winding.

Further to earlier answers, it's important to call the higher coil
resonances 'overtones' or 'modes' rather than 'harmonics'.
Remember, 'harmonics' refers to the frequency components of a 
repetitive signal, whereas 'overtones' or 'modes' refers to the
multiple resonant responses characteristic of any sort of
distributed resonator - mechanical or electrical.  Overtones are
not (except by chance!) related to one another by integer ratios.
--
Paul Nicholson,
Manchester, UK.
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